Not Applicable
Not Applicable
The present invention relates generally to the field of data networks, and in particular to techniques for emulating a Local Area Network (LAN) over a wide-area network (WAN) such as an Asynchronous Transfer Mode (ATM) network.
LAN emulation or LANE is a technique for providing data communication services between devices residing on different LAN segments that are interconnected by a long-distance or wide-area network (WAN). LANE effectively hides the underlying WAN network from the devices, and thus enables the devices to communicate using only their native LAN protocol. Several important benefits can be achieved using LANE, notably a measure of backwards compatibility. By supporting LANE, newer WAN networking equipment can successfully interoperate with devices having only LAN interfaces. Customer investments in LAN equipment can be protected, easing customer acceptance of newer networking technology.
One common environment in which LANE is used is in ATM-based networks. In ATM networks, data is transferred in the form of fixed-length cells along pre-established xe2x80x9cVirtual Connectionsxe2x80x9d or VCs. This operation is very different from the operation of most LANs. One widely used LAN protocol, for example, is the Ethernet protocol. Ethernet networks generally employ a broadcast transmission medium, such as a multi-drop coaxial cable or twisted conductor pairs. Data is sent in the form of frames, each of which includes a destination address identifying the network node to which the frame is being sent. All nodes are required to xe2x80x9clistenxe2x80x9d for transmissions that contain the address of the node as the destination address. The LANE mechanism in an ATM WAN is responsible for presenting appropriate interfaces to LAN segments connected by the WAN, and performing operations on behalf of the LAN devices using mechanisms available in the ATM network.
In particular, the LANE system is required to forward frames from one LAN segment to another across the WAN. The LANE system is responsible for learning the locations of LAN devices in the WAN, maintaining a set of mappings from LAN destination addresses to VCs by which the destination nodes can be reached, and using the mappings to forward frames through the WAN toward destination nodes. The process of learning the mappings from destination addresses to VCs is referred to as xe2x80x9caddress resolutionxe2x80x9d. Another function performed by LANE is the flooding or broadcasting of frames containing xe2x80x9cunknownxe2x80x9d destination addresses (i.e., addresses identifying nodes whose physical locations in the WAN are unknown) to all possible destination LAN segments. Once a destination address becomes known, frames are unicast transmitted using the now-known physical address information. The LANE system is responsible for ensuring that destination nodes receive sequences of frames in the same order as sent by source nodes, even in the interval surrounding a transition from broadcast to unicast servicing.
A LANE configuration for ATM is described in a specification entitled xe2x80x9cLAN Emulation over ATM Version 2xe2x80x94LUNI Specificationxe2x80x9d, published by the ATM Forum as document number AF-LANE-0084.000, July 1997. An emulated LAN or ELAN includes a group of geographically separated LANs that communicate with each other over an ATM network. The ELAN is structured according to a client-server model. LAN emulation clients (LECs) communicate with various LAN servers to carry out LANE operation. Among the LAN servers are a LAN emulation server (LES), a Broadcast and Unknown Server (BUS), and a LAN Emulation Configuration Server (LECS).
The main function of a LEC is to forward data frames across the ELAN to a peer LEC on another LAN segment. As part of this function, the LEC must learn and maintain mappings between LAN addresses (such as MAC addresses) and ATM addresses of remote LECs via which LAN destinations can be reached.
The LES enables the LEC to join an ELAN and to request an address resolution service, commonly referred to as LAN emulation address resolution protocol or LE_ARP. The BUS provides data delivery service on behalf of the LECs for broadcast, multicast, and xe2x80x9cunknownxe2x80x9d frames (i.e., frames destined for nodes whose location in the network are not known to a service-requesting LEC). The LECS provides the LECs with configuration information such as the network address of the LES, an ELAN identifier value, and an allowed maximum frame size.
In one known configuration, a LAN emulation client includes standard LEC software that is combined with special-purpose hardware that is used to send and receive frames across the ELAN. The hardware includes segmentation and reassembly (SAR) logic for transporting the frames as groups of ATM cells on the ATM network. The hardware is responsible for the frame-forwarding function when the destination address is known, i.e., when the hardware has a valid mapping between the destination address of the frame and an ATM VC on which the frame should be forwarded to a remote LEC. The standard LEC software handles all control-related transactions, such as sending LE_ARP requests to the LES, and sending unknown data frames to the BUS for broadcasting to the other LAN segments in the ELAN. One known example of such standard LEC software is embodied in a product known as the Soft-ATM(trademark) LEC Subsystem, available from Harris and Jeffries, Inc. of Dedham, Mass., USA.
Prior LANE configurations such as described above have suffered from degraded performance under certain operating conditions, especially when the volume of received unknown frames (i.e., frames including destination addresses whose mapping to VCs is unknown) is high. This condition can occur during initialization, for example, or after re-configuration of a network access device that provides LANE functionality. It is desirable to attain overall better network performance in network access devices that provide LANE functionality, even during periods of initialization or reconfiguration.
In accordance with the present invention, LAN emulation apparatus and methods are disclosed in which the broadcasting of unknown frames is handled in hardware rather than software. This results in generally improved performance, especially during initialization periods in which many destination addresses are unknown. The hardware includes a special interface to LEC software for coordinating their respective functions, especially when a destination address is listed as unknown and when it has transitioned from unknown to known.
The apparatus includes LAN interface logic that receives frames from a LAN segment, and frame transport logic that transfers frames to and from a connection-based network such as ATM. Hardware forwarding logic is used to determine whether a mapping between the destination address (DA) of the frame and a destination VC in the connection-based network exists. Frames containing a known DA are passed to the frame transport logic for transfer to the connection-based network using the VC to which the destination address is mapped. For each frame containing an unknown DA, the unknown DA is passed to a LAN emulation client (LEC) processor used to establish a mapping for the unknown destination address. The frame is simultaneously passed to the frame transport logic to be transferred to a broadcast and unknown server (BUS) in the connection-based network used to broadcast the frame on the emulated LAN.
The LAN emulation client (LEC) processor participates in a LAN emulation address resolution protocol (LE_ARP) with a LAN emulation server (LES) in the network, which is responsible for maintaining the mappings between DAs and addresses of other LECs on other network nodes via which the DAs can be reached. The LEC processor creates an LE_ARP request message containing the unknown DA, and passes this message to the frame transport logic along with an identifier of a VC of the LES. The frame transport logic sends the request message on the indicated VC. The LES responds by returning the corresponding network address in an LE_ARP reply message, which is forwarded to the LEC processor by the frame transport logic upon receipt.
Upon receiving the LE_ARP reply message, the LEC processor determines whether a new connection to the network address is needed, and establishes one if necessary. The LEC processor provides the mapping between the now-known DA and the new or existing connection to the forwarding logic for use in forwarding subsequently received frames.
The disclosed LANE system employs standard LEC software such as the above mentioned Harris and Jeffries LEC software, along with custom interfaces to account for the functionality of the forwarding logic. A dual-SAR architecture is also used. One SAR handles real-time data traffic, while the other handles protocol control traffic. In the disclosed system, the ATM network relies on a Synchronous Transfer Mode (STM) transport mechanism and a Synchronous Optical Network (SONET).
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.